A supercapacitor is a specific type of energy storage device which is getting more and more popular due to capabilities of high speed charging/discharging and high current/power output. A typical structure of a supercapacitor unit 10 of a supercapacitor is schematically shown in FIG. 1, which includes an anode 11, a cathode 12, a separator 13 and an electrolyte 14. Each of the anode 11 and the cathode 12 is made of activated carbon, which is porous so as to have relatively large area. The separator 13 is disposed between and separates the anode 11 and the cathode 12 for separation. The electrolyte 14 rinses the electrodes 11 and 12, and the separator 13. While being continuously supplied with a voltage, the electrodes 11 and 12 carry charges of different polarities, and the ions are then adsorbed onto the surfaces of the electrodes and accumulate. Afterwards, when the voltage supply is suspended, the anions and cations are released from the anode and the cathode, respectively. Meanwhile, a voltage between the electrodes 11 and 12 decreases gradually as a result of the self-discharging effect.
A supercapacitor, compared to conventional electrolytic capacitors, exhibits high charge-storage capacity due to the large overall surface area of the porous carbon material. Therefore, the charge-storage capacity of a supercapacitor can be up to thousands of the capacity of an electrolytic capacitor. Further compared to conventional lithium-ion batteries which conduct electrochemical conversion at the electrodes, the charge storage of a supercapacitor is implemented by physical adsorption. Therefore, a supercapacitor has a much higher charging/discharging speed, and is applicable to electric devices involving high current and high power output.
As described above, the operational principle of a supercapacitor is to physically adsorb ions onto electrodes for charging under a continuous voltage supply. Therefore, once the voltage supply is interrupted, the charges at the electrodes would be diminishing and the cations and anions would be desorbed from the electrodes. Then, the voltage between the electrodes would decrease gradually as self-discharging. As known, the self-discharging is relative to leakage current, and the leakage current would lower the charge-storage level of the supercapacitor. On the other hand, larger the leakage current requires more power for maintaining the voltage and results in less satisfactory electric performance of the supercapacitor.